This invention relates in general to CATV mixers and in particular to a commutating type single-balanced resistive RF mixer that is operable with high level input signals.
It is known, for example, to provide a receiver having an ultra high frequency upconverting mixer wherein a local oscillator (LO) frequency of 1.1 gigahertz is used to develop a 600 megahertz intermediate frequency (IF) signal from a 500 megahertz radio frequency (RF) input signal. In many applications, the signal to noise ratio of the receiver is critical making it desirable to pre-amplify the received RF signal to the extent that it can be handled in a linear manner by the mixer. Any nonlinearities, such as those present in conventional diode mixers, give rise to intermodulation and other forms of distortions. Because of their relatively linear channel resistance, GaAs MESFETS have been used in an unbiased drain configuration to form narrow band, single-ended resistive mixers. The solution to the problem of achieving low intermodulation distortion with high signal handling capacity in a wideband balanced mixer has, however, still eluded the art.
In an article, entitled "A GaAs MESFET Mixer With Very Low Intermodulation," by Steven A. Mass in the IEEE Transactions on Microwave Theory Techniques, Volume MTT-35, #4 April, 1987, the foregoing prior art type of resistive mixer for high frequencies is described. The mixer described has low intermodulation distortion but, since it is a single ended device, i.e., it uses a single GaAs MESFET with both RF and IF signals appearing across the drain-source terminals of the MESFET, it is usable over only a relatively narrow frequency range, thereby limiting its use in many applications.
Another article, entitled "High Dynamic Range Mixing With the Si8901," by Ed Oxner in the March, 1988 issue of hr (ham radio), shows a doubly balanced commutation-type MOSFET mixer. The mixer is constructed in a bridge configuration with four MOSFETS and is usable over a relatively wide frequency range. This mixer suffers a disadvantage in that the source terminals of the MOSFETS cannot be connected to ground. Furthermore, the symmetric LO source feeding the gate electrodes is not an ideal floating source. Consequently, the control voltage applied to the gates is a combination of the LO voltage and the RF or IF voltage and the mixer switching action can be readily affected by signals other than the LO signal, which is certainly not an ideal situation. A similar mixer is shown in Siliconix Application Note AN85-2, "A Commutation Double-Balanced MOSFET Mixer of High Dynamic Range", January 1986.
The commutating single-balanced resistive mixer of the invention includes a very tightly coupled trifilar balun transformer for commutating the RF signal into the IF port and includes a pair of GaAs MESFETS for high speed, switching. The mixer has a relatively low conversion loss (about 6 db), good 50 ohm matching at both the RF and IF ports, a two tone third order intercept of about +36 dbm to +40 dbm and is capable of converting an RF frequency range of about 50-550 MHz into an IF frequency of about 600 MHz.